How to multiply an integer with a list of integers?

To concatenate all the elements you can iterate through the list and use the .append(Int n)StringBuilder method .

At the end of the tour, the StringBuilder will have as value the concatenation of the digits of the numbers and you can create an Integer usingInteger.valueOf(String string)

The operation valueOfis not safe, it can throw NumberFormatException for strings like "12a1" or "12-12", so if you had negative values ​​you would get such an exception.

In turn, as @Arie CwHat comments in @Einer's answer , creating a from a variable number of digits and potentially greater than the maximum precision is not a good idea.Integer

To avoid this problem we can use BigInteger, which does not have a defined maximum precision but can grow from -2^Integer.MAX_VALUE(exclusive) to 2^Integer.MAX_VALUE(inclusive), where Integer.MAX_VALUE = 2^32 - 1(Extracted from Is there a upper bound to biginteger? )

   public static BigInteger listMultiplication(List<Integer> list, int k){
        StringBuilder builder = new StringBuilder();
        for (Integer num : list) {
            builder.append(num);
        }
        try{
            System.out.println("Multiplicando "+builder.toString()+ " por "+k);
            BigInteger concatNumber= new BigInteger(builder.toString()); 
            return concatNumber.multiply(BigInteger.valueOf(k)); 

        }catch (NumberFormatException e) {
            System.out.println("Formato numérico invalido: "+builder.toString());
            return null;
        }
    }

List<Integer> numbers = Arrays.asList(2, 8, 9, 1, 4, 0, 6);
System.out.println(listMultiplication(numbers, 1));
System.out.println(listMultiplication(numbers, 2));
System.out.println(listMultiplication(numbers, 10));

Multiplying 2891406 by 1 2891406

Multiplying 2891406 by 2 5782812

Multiplying 2891406 by 10 28914060

I do not want to add all the elements of the list, I want to join them, that is, to form a single number with all of them

Well then you will have to concatenate each number in the list and then convert it to double:

List<Integer> lista=new Arraylist<>();
lista.add(2);
// ..
//..

String numeroString = "";
for(int numero : lista)
{
  numeroString += String.valueOf(numero);
}

double resultadoNumeroConcatenado = Double.parseDouble(numeroString);
double resultado = k * resultadoNumeroConcatenado

Concatenating a list of nelements it is very likely that this maximum Integer value will be exceeded, so I use it doubleto avoid a stack overflow due to the Integer limit of 2 147 483 648nine digits.

Update:

You can also use the class BigIntegerthat does not have a defined limit like double:

//..
BigInteger numeroBigInt = new BigInteger(numerosString);
BigInteger resultado = numeroBigInt.multiply(new BigInteger(String.valueOf(k)));

This is an example of how you could do it: (but there are many other ways)

import java.util.ArrayList;

public class Main {

    public static void main(String[] args) {
        ArrayList<Integer> lista = new ArrayList<Integer>();

        lista.add(1);
        lista.add(2);
        lista.add(3);
        lista.add(4);
        lista.add(2);

        int potencia = 0, suma = 0, k = 2;
        for (int i = lista.size() - 1; i >= 0; i--, potencia++) {
            // esto es igual a lista[lista.largo - indice] * 10^indice, 
            // pero separo potencia e indice para que quede claro
            // suma += lista.get(i) * Math.pow(10, lista.size() - i);

            suma += lista.get(i) * Math.pow(10, potencia);
        }

        // antes de este print el resultado es 
        // suma = (1 * 10^4) + (2 * 10^3) + (3 * 10^2) + (4 * 10^1) + (5 * 10^0)
        System.out.println(suma * k);
    }
}